It would be wrong to round up and exterminate a billion people in order to ensure than one billion and one babies are born.
I agree, but I don't think that cuts to the point. The process of rounding up and killing a billion people, the sadness of people left behind, the skill loss, and the change of the age distribution, would all have large negative effects, and a billion and one babies would be a heck of a baby boom.
While practical issues mean that killing people is just about never the right thing to do, I don't agree that "creating new people is much less valuable than preserving old ones". See my response to Nisan.
This perspective looks deeply insane to me.
I would not kill a million humans to arrange for one billion babies to be born, even disregarding the practical considerations you mentioned, and, I suspect, neither would most other people. This perspective more or less requires anyone in a position of power to oppose birth control availability, and require mandatory breeding.
I would be about as happy with a human population of one billion as a hundred billion, not counting the number of people who'd have to die to get us down to a billion. I do not have strong preferences over the number of humans. The same does not go for the survival of the living.
Imagine you accept the main idea of cryonics, that if we freeze brains future technology is likely to be able to extract the encoded information and revive the person digitally. [1] While this currently costs about $120K/person, if we did it routinely to everyone it could probably get down below $1000/person. Which is interesting: the current cost of averting a death is around $1700, but someone who doesn't die of malaria is still going to die of old age, so you can't really say their death was "averted". While someone who is revived after being frozen wouldn't live eternally, they might get to experience thousands of years of subjective life. In terms of life-years, getting cryonics to be really cheap and paying for people to get it sounds like it beats GiveWell's top charities.
Aside from not agreeing that cryonics is likely to work, however, I don't think the value is actually all that high. A future world which would revive large numbers of people we freeze today would be massively different from the current world economically, but would still have constraints. There would be some number of digital people that could run simultaneously on whatever people-emulating hardware they have. Preserving additional 21st century minds would give future people the option to run revived people instead of new people, but I don't think it affects the overall number emulated.
[1] I don't think this is likely.
I also posted this on my blog.